1. Field of the Invention
The present invention relates to a droplet discharge control method, and a liquid discharge apparatus, and more particularly to a droplet discharge control technique which forms a favorable image, drawing, or another shape with consideration for the effect of mutual interference of the droplets as they land.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. An inkjet printer forms data on recording paper by driving recording elements (nozzles) of a recording head in accordance with data, thereby causing ink to be ejected from the nozzles.
In an inkjet printer, a recording medium and a recording head with a large number of nozzles are moved in a relative fashion, and a desired image is formed on the recording medium by ejecting ink droplets from the nozzles.
There is a demand for high-speed and high-quality printing in inkjet recording apparatuses, and high-speed printing is achieved by reducing the ink droplet discharge cycle, conveying the recording medium at a large velocity, and performing other techniques.
In order to print high-quality images, on the other hand, high-quality half-toning and higher resolution is achieved by making the dots that form the image more minute and ensuring higher density. Making the dots smaller is achieved by discharging ink in small amounts, for example, and increasing the density of the dots by forming with greater density the nozzles which eject ink. When the dots are made more highly dense, the spacing between dots that are formed in mutually adjacent positions results in an overlapping formation area thereof.
In a row of dots formed so that a plurality of ink droplets land substantially simultaneously and mutually overlap, a phenomenon (landing interference) occurs in which the ink droplets that have landed on the surface of the recording medium join with each other and are drawn to the vicinity of the center area, and dots with a prescribed size are not formed.
Therefore, in order to inhibit such landing interference, droplet discharge control is carried out so as to impart a time difference to the dots which are formed by droplet discharge, wait for the previously landed ink droplet to permeate the recording medium, and then discharge the ink droplet that will land subsequently.
However, when droplets are simultaneously discharged to a line in the main scanning direction with a line-type head to form a single line in the main scanning direction, it is difficult to impart a time difference to the droplet discharges that form adjacent dots.
The related art is more specifically described below with reference to FIGS. 12A to 14D.
FIGS. 12A and 12B are diagrams that describe the discharge control in the inkjet recording apparatus according to the related art.
FIG. 12A shows a row 200 of dots formed on the same line in main scanning direction, sub-scanning direction, or another direction using an inkjet recording apparatus. In the row 200 of dots, dots 202, 204, and 206 with the same diameter D are aligned with the same pitch P between the dots. The droplets land so that the droplets discharged to mutually adjacent droplet discharge points on the recording paper overlap (that is to say, so as to satisfy the relation D>P).
FIG. 12B shows the shape of the ink droplet 210 after landing when ink droplets which form the dots in the row 200 of dots shown in FIG. 12A are discharged simultaneously (with the same timing). The ink droplets that form the dots of the row 200 of dots come together when the ink in the center area is drawn by mutual interference (landing interference) at the time of landing. This is a phenomenon that occurs due to the fact that the ink droplets (liquid droplets) have a characteristic whereby they take a round spherical shape due to the effect of the surface tension of the ink.
Therefore, the density of the ink increases in the vicinity of the center area, the width of the center area increases in comparison with both ends, and when the combined ink droplet 210 is fixed on the recording paper, the width h in the center area is h=D′ (where D′>D), and the line width at both ends is h=D″ (where D″<D), forming a nonuniform line drawing (row 200 of dots). As used herein, the term “width at both ends” refers to the width of the dots formed by the ink discharged to the landing points at both ends.
In the present specification, the term “dot” refers to a substantially circular point formed when an ink droplet discharged onto the recording paper is fixed on the recording paper. When the shape of the ink droplet breaks down and a shape that is different from a substantially circular shape is formed, or when a shape is formed that differs from a shape formed by the overlapping of a plurality of substantially circular shapes due to phenomena such as the grouping of a plurality of ink droplets, the shape formed by the discharge point is also referred to as a dot.
The aspect in which droplets are discharged with the same timing may include an aspect in which droplets are discharged at short time intervals, and an example of short time intervals is time in which the permeation time of the ink is 1% or less.
A more specific example is shown in FIGS. 13A to 13D and FIGS. 14A to 14D. It should be noted that in FIGS. 13A to 13D and FIGS. 14A to 14D, the same key symbols are assigned to the same or similar portions as in FIGS. 12A and 12B, and a description thereof is omitted.
FIG. 13A shows a dot 202 with a diameter D=30 μm formed by an ink droplet discharge amount of 2 pL, and FIGS. 13B to 13D show line drawings (rows of dots) 212 to 216 that are formed when three or more ink droplets are discharged on the same line with the following discharge conditions: an ink amount of 2 pL per single discharge, a dot diameter D=30 μm when the dots are formed singly by the ink droplets, a pitch of P=10 μm between the dots, and simultaneous droplet discharge (delay time of the droplet discharges is 1% of the ink droplet permeation time or less).
FIG. 13B shows a line drawing 212 formed when five ink droplets are simultaneously discharged under the above-described conditions, the ink at both ends is drawn to the ink in the center area, the width hc in the center area is 40 μm, which is wider than the prescribed width of 30 μm, and the width he at both ends is 20 μm, which is less than the prescribed width. Also, the density in the center area is higher than the prescribed density, and the density at both ends is less than the prescribed density.
FIG. 13C shows a line drawing 214 formed when ten ink droplets are simultaneously discharged under the above-described conditions, and FIG. 13D shows a line drawing 216 formed when 60 ink droplets are simultaneously discharged under the above-described conditions.
In the line drawing 214 formed with 10 ink droplets, the width hc in the center area side is 45 μm, which is greater than the prescribed width, the width he at both ends is 20 μm, which is less than the prescribed width, and since the width of the line drawing 214 sequentially changes from the center area toward both ends, the slope (percentage that the width changes) is substantially fixed, as shown in FIG. 13C.
In the line drawing 216 formed with 60 ink droplets, the width hc in the center area side is 50 μm, which is greater than the prescribed width D, and the width he at both ends is 15 μm, which is less than the prescribed width, as shown in FIG. 13D.
FIGS. 14A to 14D show the three-dimensional shape (cross-sectional shape) of the ink that forms the dot 202 and line drawings 212, 214, and 216 shown in FIGS. 13A to 13D.
FIGS. 14B to 14D show the cross-sectional shapes of the line drawings 212, 214, and 216 which are all half oval shapes with the center portion serving as the apex such that the height decreases from the center area to both ends.
In this manner, when three or more dots with the same size are formed on the same line by simultaneously discharged droplets so that the droplets overlap on the recording paper at the time of landing, the droplet amount in the center area becomes greater than the droplet amount at both ends, the width of the center area increases in comparison with the width at both ends, the density increases, a nonuniform line drawing is formed, and the image quality is reduced.
It should be noted that the numerical values shown in FIGS. 13A to 13D are merely examples, and the indicated values may vary depending on the type of ink, type of recording paper (recording medium), and combinations thereof.
The recording method and apparatus cited in Japanese Patent Application Publication No. 9-272226 are configured so that the original recording data and interpolation data are output during separate head scans and recording is performed in order to prevent mutual interference of adjacent dots, and ink bleeding and mixing between the inks are prevented.
However, the discharge timing can be controlled so as to wait for the ink droplet that previously landed to permeate (to become fixed to) the medium and to then discharge the ink droplet that will land next in order to solve the problems of the related art shown in FIG. 12A to FIG. 14D, but the discharge cycle cannot be reduced with control in which the subsequent ink lands after the previously landed ink droplet completes permeation. Also, since the permeation time of the ink (permeation velocity) varies depending on the combination of ink and recording medium, control must be carried out so as to vary the discharge cycle in accordance with the type of ink and recording medium that is to be used, and the control load of the apparatus is increased.
In the recording method and apparatus thereof cited in Japanese Patent Application Publication No. 9-272226, the original recording data scan and the interpolation data scan must be separately carried out when dots are formed with high density, a reduction in productivity is unavoidable, and it is difficult to achieve both higher image quality and higher printing speed.